The disposal of large volumes of refinerygenerated oily sludge is a costly operation. The destabilization of this sludge to separate the oil, water, and solid phases would greatly reduce the volume for disposal and would represent significant oil recovery. The objective of this invention is to destabilize the sludge by the application of microemulsion technology.
Microemulsions are translucent or transparent, thermodynamically stable mixtures of an oil, an aqueous phase and one or more surfactants chosen to impart certain phase behavior characteristics. Unsaturated, single phase microemulsions will spontaneously imbibe more oil and/or water until they become saturated. Microemulsions saturated with oil exist as a lower water-continuous microemulsion containing essentially all the surfactants in equilibrium with an excess oil phase on top. Microemulsions saturated with water exist as an upper oil-continuous microemulsion containing essentially all the surfactants in equilibrium with an excess aqueous phase. Microemulsions saturated with both oil and water exist as a middle bicontinuous microemulsion in equilibrium with both excess oil and aqueous phase. Interfacial tensions between saturated microemulsions and excess oil and/or aqueous phases are ultra-low, often less than 0.01 dyne/cm. The relative amounts of oil and water held in saturated microemulsions depends on oil and aqueous phase compositions, temperature and the hydrophilic vs. lipophilic tendencies of the surfactant blend.
The surfactant HLB (hydrophilic-lipophilic balance) is one way of defining the hydrophilic vs lipophilic tendencies of the surfactant in water and Nujol mixtures at room temperature. A better way which takes account of variations in temperature and aqueous and oil phase compositions is microemulsion phase behavior. Hydrophilic surfactants form lower phase microemulsions with the given oil and aqueous phase; lipophilic surfactants form upper phase microemulsions. Mixtures of hydrophilic and lipophilic surfactants can form middle phase microemulsions when their ratio is properly adjusted. The relative amounts of oil and water in middle phase microemulsions at a given temperature depends on the weight ratio of hydrophilic/lipophilic surfactants (H/L ratio). When the H/L ratio is so adjusted that the microemulsion contains equal amounts of water and oil, the microemulsion is said to be balanced and the surfactants are at balance for the given temperature and aqueous and oil compositions. The absolute amount of water and oil at balance is a measure of surfactant efficiency i.e. solubilizing power.
We have discovered that a surfactant blend which forms an upper phase microemulsion when contacted with refinery sludge containing oil, water and solids breaks the sludge and sheds water leaving a residue of oily solids. Likewise, a surfactant blend which forms a lower phase microemulsion causes oil to be shed from refinery sludge leaving a residue of solids with much reduced oil content. The oil which is shed from the sludge remains dispersed in the aqueous phase as a water-continuous microemulsion and/or forms a separate oil phase floating on top. Surfactant blends forming middle phase microemulsions cause both oil and water to be shed from refinery sludge.
In an alternate embodiment of the invention of using microemulsion phase behavior to formulate surfactants for breaking refinery sludge, unsaturated water-continuous microemulsions extract water causing oil and solids to separate from the sludge. With proper choice of surfactant, the extracted water may be separated from the microemulsion by heating the microemulsion thus restoring the water-continuous microemulsion to its unsaturated condition. The remaining oily solids are than contacted with an unsaturated oil-continuous microemulsion which extracts oil from the solids. Temperature adjustment allows the separation from the microemulsion of the extracted oil.